Electrophotographic printing is well known and has been widely refined. For example, today, almost every office and indeed some homes have electrophotographic copiers. The industry has grown to the point where it is now a highly competitive multi-billion dollar industry. In most instances, these home and office copiers are capable of providing only about a few copies per minute.
In electrophotography, images are photoelectrically formed on a photoconductive layer mounted on a conductive base. Liquid or dry developer or toner mixtures may be used to develop the requisite image.
Liquid toner dispersions for use in the process are formed by dispersing dyes or pigments and natural or synthetic resin materials in a highly insulating, low dielectric constant carrier liquid. Charge control agents are added to the liquid toner dispersions to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
The photoconductive layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting or alternatively by writing over the surface with a laser, L.E.D., or the like. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light with an electrostatic charge remaining in the image area. The charged pigment and/or dye particles from the liquid toner dispersion contact and adhere to the image areas of the photoconductive layer. The image is then transferred to the desired substrate, such as a travelling web of paper or the like.
In contrast to office and home copiers, high speed electrophotographic printing presses are being developed wherein successive images are rapidly formed on the photoconductive medium for rapid transfer to carrier sheets or the like travelling at speeds of greater than 100 ft./min. and even at speeds of from 300-500 ft./min.
A development shoe or electrode is spaced close to the photoconductive surface and acts as a reservoir holding the liquid toner dispersion for application thereof to the photoconductive layer. In one type of electrophotographic printing system, the image portion of the photoconductive layer has a charge of high potential and given polarity with the non-image areas, due to exposure thereof, carrying a charge potential of lesser magnitude than the image area charge but of common polarity therewith. The solids, color imparting particles in the liquid toner dispersion comprise a charge of opposite polarity. Accordingly, an electrical field is created from the image areas to the non-image areas with the oppositely charged solids, color-imparting particles of the toner dispersion rapidly migrating in the opposite direction, i.e., toward the image areas.
The developer shoe or electrode is provided with an electrical charge having a potential intermediate that supplied to the image and non-image areas and having a common polarity with those area charges. Thus, an electrical field is created in the direction of the developer electrode to the non-image areas with toner particles located in the non-image areas being drawn to the developer electrode. In normal office or home photocopiers, in order to inhibit agglomeration of the charged solids toner particles on the developer shoe, periodically, for instance, between running of each individual copy, a reversing circuit reverses the charge polarity on the developer electrode.
Unfortunately, in high speed electrophotographic copiers of the type herein contemplated, the process cannot be frequently interrupted if the high speed goals of the press are to be attained. Accordingly, there is a need in the art to provide a mechanism in which toner particle agglomeration on the developer shoe can be inhibited or minimized without shutting down or frequently interrupting the continuous process.
Further, due to the high speed nature of the process, it is necessary to ensure that a sufficient supply of liquid toner dispersion is applied uniformly and in sufficient quantity to the rapidly rotating (e.g., 100 ft./min-500 ft./min. peripheral speed) photoconductive cylinder. This is especially important in that the solids particles of the dispersion are rapidly depleted in order to form the required image as the cylinder continues its rapid rotation.
Moreover, it is also desirable to provide for a movable mounting of the developer electrode in its position next to the rotating cylinder so that, if necessary, during machine down-time, the electrode can be easily displaced from its normal operating position to facilitate repair and cleaning thereof.
In addition to the above, it is highly desirable to provide a reverse roller downstream from the development electrode that acts to shear excess toner from the rotating photoconductive drum. This is important since excessive amounts of toner on the cylinder that are subsequently transferred to a travelling carrier web or the like will result in inordinate amounts of noxious vapors being released in downstream drying and fusing operations, and possibly to the surrounding environment. For this reason, it is desirable to provide a sensor or the like in operative association with the reverse roller that will sense when same is not in its proper position, and send a signal to the machine operator or prohibit rotation of the electrophotoconductive drum altogether.